Rows Read Versus Rows Returned

The visual of the arrows in an execution plan strongly suggests that they represent the flow of rows from one operator to another. And hence, the width of that arrow strongly appears to be an indication of how many rows are passed between the connected operators. That used to be always the case. But unfortunately, this changed in December 2017 (or earlier, but that was when I first noticed the change).

The root cause is the addition of a new property, returned by Scan and Seek operators: Number of Rows Read (as well as its estimated counterpart: Estimated Number of Rows to be Read). Now don’t get me wrong, those properties are awesome and I’m super happy that they were added to execution plans. When a filter condition is pushed into a scan or seek operator as a Predicate property, I want to be able to see how effective it is, and comparing Actual Number of Rows to Number of Rows Read (or Estimated Number of Rows to Estimated Number of Rows to be Read) helps me assess just that. Which in turn can help me decide whether I should change my indexing, or try to rewrite the query. But I digress.

Click through to learn what the issue is. Hugo describes a tricky situation where there are two valuable measures but only one way to show them. If you agree with Hugo’s preferences, here’s a Feedback item for you.